1. Field of the Invention
The present invention relates to a method of identifying a semiconductor wafer in fabrication processes utilizing a bar code pattern formed theron, and a method for manufacturing a semiconductor device. More particularly, this invention relates to obtaining a better contrast in detecting a bar code pattern, and is especially suitable for the fabrication processes of semiconductor wafers, in which a different metal pattern of an integrated circuit is required after the same prior bulk processes are finished.
2. Description of the Prior Art
When semiconductor wafer are planned for fabrication of gate array integrated circuits, the bulk wafer processes are the same for steps prior to the first metallization process. Because such wafers are subjected to a different patterning process for a metal layer depending on a designed logic circuit, it is required that a code pattern for each circuit be formed at a time just before the patterning process for the first metallization.
Generally, identifying an individual wafer is not necessary for a typical process of the integrated circuits. Because many wafers in a same lot, which are usually carried in the same wafer carrier, are subject to the same process, a lot identification of wafers is enough to identify the process. However, in a metallization process of gate array integrated circuits, a production quantity using the same metal pattern is extremely limited. Therefore, it is necessary that each wafer should be individually identified in every step on and after the first metal patterning process.
With the progress of automation of production, a bar code pattern is more practical because it is suitable for an automatic recognition utilizing a bar code reader. The conventional method of forming a bar code pattern for identification of individual wafers is to utilize a photolithography technique for the metal layer, which is separately patterned for a specific requirement.
The bar code pattern thus formed in the metal layer is subsequently subjected to the formation processes of an insulating layer, a second metallization when required, a passivation layer, and the like. In each process, the identification of each wafer in the prior art utilizes the method of applying light to the surface of the wafer from the front side thereof, receiving the reflected light, and detecting the bar code pattern.
When the bar code pattern is formed on the wafer surface itself, the problem is more serious. Because the deposited layers on the bar code pattern layer have different reflection factors and flat surfaces so as to cause a multiple reflection of the light for detection, it is difficult to get a clear and distinct detection signal from the bar code pattern. Furthermore, the bar code pattern is liable to be damaged during the wafer processes so that the bar code may be incorrectly identified. Sometimes it becomes impossible to decode the signal. Even for a case in which the naked eye can recognize the code pattern, an automatic identification thereof can become impossible.